Tool-to-tool matching control method and its system for scanning electron microscope

ABSTRACT

A system for controlling a tool-to-tool disparity between a plurality of scanning electron microscopes includes a measuring unit for measuring a tool-to-tool disparity between plural scanning electron microscopes based on information extracted from secondary electron images which are captured by imaging a reference pattern, a tool state monitoring unit for monitoring tool states of each of the plural scanning electron microscopes, and an output unit for displaying on a screen a relationship between the tool-to-tool disparity between the plural scanning electron microscopes and tool states of each of the plural scanning electron microscopes monitored by the tool state monitoring unit. The tool state monitoring unit monitors the tool states of each of the plural scanning electron microscopes while imaging the reference pattern by using each of the plural scanning electron microscopes.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/349,751, filed Jan. 7, 2009, now U.S. Pat. No. 8,003,940 which is acontinuation of U.S. application Ser. No. 11/583,886, filed Oct. 20,2006, now U.S. Pat. No. 7,476,857, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a scanning electron microscope formeasuring micropattern dimensions, and more particularly to a scanningelectron microscope system having a function to control, or correct, adisparity in dimension measurements between measuring scanning electronmicroscopes, as well as controlling, or correcting, a variation indimension measurements obtained by a microscope over time. (Such adisparity or variation may be hereinafter referred to as a “tool-to-tooldisparity”). The present invention also relates to a tool-to-toolmachine control system including scanning electron microscopes and atool-to-tool matching control apparatus for scanning electronmicroscopes having such a function.

As semiconductor device patterns have been scaled down in semiconductormanufacturing processes, there has been a need for a dimension measuringapparatus having higher measurement accuracy. The demands related tomeasurement accuracy include increasing the measurement accuracy ofindividual measuring apparatuses, reducing disparities in dimensionmeasurements between a plurality of measuring apparatuses provided in aproduction line, and reducing variations in dimension measurementsobtained by a measuring apparatus over time.

Scanning electron microscopes (SEMs) for measuring micropattern width,referred to as “length measuring SEMs” or “critical dimension SEMs” (or“CDSEMs”), have been used to measure the width of micropatterns on theorder of a few tens of nanometers. These scanning electron microscopesare capable of capturing a micropattern image at a magnification of afew tens of thousands of times to three hundred thousand times.

Attempts have been made to correct or reduce tool-to-tool disparities indimension measurements between such measuring scanning electronmicroscopes or correct or reduce variations in dimension measurementsobtained by such a scanning electron microscope over time by correctingthe measured values themselves, instead of calibrating the microscopes.

Specifically. Japanese Laid-Open Patent Publication No. 5-248843 (1993)discloses a scanning electron microscope that performs the followingsteps: for each test sample under each test measurement condition (ateach different magnification), finding a correction equation y=a*x+bbased on the design value and length measurements; saving these foundcorrection equations from the terminal unit to a disk storage device;and when an actual sample is measured under a given measurementcondition, retrieving the correction equation matching this sample andmeasurement condition, and correcting length measurements using theselected correction equation.

SUMMARY OF THE INVENTION

Thus, in order to reduce disparities in dimension measurements between aplurality of measuring scanning electron microscopes or reducevariations in dimension measurements obtained by a scanning electronmicroscope over time, the technique disclosed in the above patentpublication corrects the measured values of pattern dimensions (insteadof calibrating the microscopes). The reason for this is that scanningelectron microscopes are very sophisticated devices and hence it is noteasy to pinpoint a factor that has caused a tool-to-tool disparity of ananometer or less, which is currently being demanded. In reality,however, since the appropriate correction method for dimensionmeasurements may vary depending on the factor that has caused thetool-to-tool disparity, pattern dimension correction cannot be properlycarried out without knowing this factor.

The present invention relates to a tool-to-tool matching control systemfor scanning electron microscopes and a method therefor capable ofquickly estimating a factor(s) that has caused a variation (ortool-to-tool disparity) in dimension measurements between a plurality ofmeasuring scanning electron microscopes or a variation in dimensionmeasurements obtained by a scanning electron microscope over time, andcalibrating these scanning electron microscopes based on the estimationresults to reduce such a tool-to-tool disparity in order to allow highlyaccurate control of the dimensions of patterns.

The present invention provides a system and method for controlling avariation in dimension measurements (or a tool-to-tool disparity)between a plurality of scanning electron microscopes for patterndimension measurement, the system comprising: measuring means for, atregular intervals, measuring a tool-to-tool disparity betweenmicroscopes based on secondary electron image, and, measuring indicatorsindicating states of the microscopes (that is, image features in thesecondary electron image data and/or device state parameters of themicroscopes), the secondary electron image data being captured byimaging a reference wafer by use of the microscopes; a tool-to-tooldisparity causing factor analyzing unit for analyzing a relationshipbetween the tool-to-tool disparity and the values of the indicatorsmeasured by the measuring means to estimate a factor that has caused thetool-to-tool disparity; and output means for displaying and outputtingthe tool-to-tool disparity causing factor estimated by the tool-to-tooldisparity causing factor analyzing unit.

Further, the present invention also provides a system and method forcontrolling a variation (or tool-to-tool disparity) in dimensionmeasurements obtained at a series of times (over time) by a scanningelectron microscope for pattern dimension measurement, the systemcomprising: measuring means for measuring a variation (or tool-to-tooldisparity) in dimension measurements obtained based on secondaryelectron image data captured by imaging a reference wafer at a series oftimes by use of a scanning electron microscope, and further measuringindicators indicating states of the microscope (that is, image featuresin the secondary electron image data and/or device state parameters ofthe microscope); a tool-to-tool-disparity causing factor analyzing unitfor analyzing a relationship between the variation and the values of theindicators measured by the measuring means to estimate a factor that hascaused the variation, the indicators indicating the states of themicroscope; and output means for displaying and outputting the variationcausing factor (or tool-to-tool disparity causing factor) estimated bythe tool-to-tool disparity causing factor analyzing unit.

Further, the present invention also provides a system and method for:storing in a database a relationship between candidate tool-to-tooldisparity causing factors and device control parameters for adjustingthe candidate tool-to-tool disparity causing factors; based on therelationship stored in the database, selecting an appropriate devicecontrol parameter for an estimated tool-to-tool disparity causingfactor; and automatically adjusting the selected device controlparameter for each microscope by an appropriate amount to reduce thetool-to-tool disparity between the microscopes.

Thus, the present invention provides a tool-to-tool matching controlsystem for scanning electron microscopes and a method therefor capableof quickly estimating a factor(s) that has caused a variation (ortool-to-tool disparity) in dimension measurements between a plurality ofmeasuring scanning electron microscopes or a variation in dimensionmeasurements obtained by a scanning electron microscope over time, andadjusting these scanning electron microscopes based on the estimationresults to reduce such a tool-to-tool disparity. This leads to highlyaccurate control of the dimensions of patterns and hence to enhancedperformance of the product.

These and other objects, features and advantages of the invention willbe apparent from the following more particular description of preferredembodiments of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing the configuration of a firstexemplary tool-to-tool matching control system including scanningelectron microscopes according to a first embodiment of the presentinvention.

FIG. 2 is a diagram showing an exemplary configuration of the individualscanning electron microscopes shown in FIG. 1.

FIG. 3 is a diagram showing an exemplary configuration of thetool-to-tool matching control apparatus shown in FIG. 1.

FIG. 4 is a flowchart showing an exemplary entire process sequence foridentifying a factor that has caused a tool-to-tool disparity accordingto the first embodiment of the present invention.

FIG. 5 is a flowchart specifically showing step S42 of FIG. 4,illustrating a sequence for capturing secondary electron image data byuse of a reference wafer for dimension measurement.

FIG. 6 is a flowchart specifically showing step S43 of FIG. 4,illustrating a sequence for calculating a CD value from secondaryelectron image data in order to estimate a factor that has caused atool-to-tool disparity.

FIG. 7A shows an SEM image of a pattern to be measured; FIG. 7B showsthe waveform of a signal obtained as a result of scanning the SEM imagealong a line in the X-direction; and FIG. 7C shows a image profileobtained by smoothing the waveform of the single-line signal.

FIG. 8 is a diagram showing an exemplary icon (or button) displayed bythe input/output unit of each scanning electron microscope of thepresent invention, wherein the button is pressed by the user to obtaindata for evaluation of tool-to-tool disparity causing factors.

FIG. 9 is a diagram showing an exemplary screen with exemplary icons (orbuttons) thereon displayed by the input/output unit of a tool-to-toolmatching control apparatus of the present invention before evaluation oftool-to-tool disparity causing factors, wherein: the screen is used tospecify a scanning electron microscope(s) and data (device controlparameter data, etc.) for evaluation of tool-to-tool disparity causingfactors; and the button is pressed by the user to start the evaluation.

FIG. 10 is a diagram showing an exemplary output screen and exemplaryinput/output screens displayed by the tool-to-tool matching controlsystem including scanning electron microscopes according to the firstembodiment of the present invention, wherein: the output screen showsevaluation results of tool-to-tool disparity causing factors; and theinput/output screens are used to automatically adjust the scanningelectron microscopes based on information about candidate tool-to-tooldisparity factors so as to reduce the tool-to-tool disparity.

FIG. 11 is a diagram showing the configuration of a second exemplarytool-to-tool matching control system including scanning electronmicroscopes according to the first embodiment of the present invention.

FIG. 12 is a diagram showing the configuration of a third exemplarytool-to-tool matching control system including scanning electronmicroscopes according to the first embodiment of the present invention.

FIG. 13 is a diagram schematically showing the configuration of thescanning electron microscope system shown in FIG. 12.

FIG. 14 is a diagram showing the configuration of a fourth exemplarytool-to-tool matching control system according to the first embodimentof the present invention.

FIG. 15 is a diagram showing an exemplary input screen used by thetool-to-tool matching control system shown in FIG. 14.

FIG. 16 is a diagram showing the configuration of a fifth exemplarytool-to-tool matching control system including scanning electronmicroscopes according to a second embodiment of the present invention.

FIG. 17 is a diagram showing an exemplary configuration of theindividual scanning electron microscopes shown in FIG. 16.

FIG. 18 is a diagram showing an exemplary configuration of thetool-to-tool matching control apparatus shown in FIG. 16.

FIG. 19 is a flowchart showing an exemplary entire process sequence foridentifying a factor that has caused a tool-to-tool disparity accordingto the second embodiment of the present invention.

FIG. 20 is a diagram showing an exemplary output screen and exemplaryinput/output screens displayed by the tool-to-tool matching controlsystem including scanning electron microscopes according to the secondembodiment of the present invention, wherein: the output screen showsevaluation results of tool-to-tool disparity causing factors; and theinput/output screens are used to automatically adjust the scanningelectron microscopes based on information about candidate tool-to-tooldisparity causing factors so as to reduce the tool-to-tool disparity.

FIG. 21 is a diagram showing an exemplary screen for specifying a datastorage method (or data storage locations) when entering dimensionvalues (or CD values) of a target pattern measured by a plurality ofscanning electron microscopes of the present invention and variousdevice state parameters.

FIG. 22 is a diagram showing an entire process sequence for correcting atool-to-tool disparity according to a sixth embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

There will now be described scanning electron microscopes (lengthmeasuring SEMs or CDSEMs), tool-to-tool matching control systemscontaining scanning electron microscopes, and tool-to-tool matchingcontrol apparatuses for scanning electron microscopes according to thepresent invention with reference to the accompanying drawings.

The present invention relates to a scanning electron microscope systemincluding a plurality of scanning electron microscopes for measuring thedimensions of a micropattern based on a captured image of themicropattern. An object of the present invention is to reduce variationsin dimension measurements between these measuring microscopes. It shouldbe noted that a variation in dimension measurements (namely, CD values)between measuring microscopes is hereinafter referred to as a“tool-to-tool disparity.”

First Embodiment

It is not easy to pinpoint a factor that has caused a variation indimension measurements between measuring scanning electron microscopes(length measuring SEMs or CDSEMs) of the present invention, since theyhave a sophisticated configuration.

A first embodiment of the present invention has been devised to addressthis problem. The first embodiment of the present invention ischaracterized in that image features in secondary electron image datacaptured by scanning electron microscopes are used as indicators forindicating states of the microscopes in order to evaluate, or examine,tool-to-tool disparity causing factors. Specifically, to evaluatetool-to-tool disparity causing factors, for example, the presentembodiment monitors the following image features in secondary electronimage data captured by microscopes: image brightness information Ca (forexample, average gray value); image contrast Cb (for example, thedifference between the highest and lowest gray values); image noiselevel Cc; tilt indicator value Cd; pitch Ce of a repeated pattern;resolution evaluation indicator value Cf calculated based on the image;and axial alignment (astigmatic adjustment) indicator value Cgcalculated based on the image. The values of these monitored featuresare compared with the dimension values (CD values) measured by themicroscopes to determine their correlation or perform principalcomponent analysis in order to identify a factor(s) that has caused thetool-to-tool disparity. Factors causing a tool-to-tool disparityinclude: image brightness difference Fa; image contrast difference Fb;image noise level difference Fc; relative tilt angle Fd; imagemagnification difference Fe; resolution difference Ft and axialalignment (astigmatic adjustment) accuracy difference Fg. Then, forexample, the following device control (or adjustment) parameters of eachmicroscope may be automatically corrected based on the comparisonresults: the photomultiplier tube gain Pa and probe current Pb; theprobe current Pc; the tilt control value of the deflection coil andstage tilt angle (or stage installation angle), Pd; the deflection coilmagnification control value Pe; the objective lens control value,retarding control value, and aperture (changed by replacing themechanism), Pt and the axial alignment coil (astigmatic adjustment coil)control value Pg. Specifically, a tool-to-tool disparity correctionprocess may be performed as follows. Each microscope captures an imageof a repeated pattern, and the pitch Ce of the pattern is measured fromthe image. Then, the correlation between the dimension measurements,namely, CD values, obtained by the microscopes and the measured valuesof the pitch Ce is calculated. If this correlation is high, a factorthat has caused the tool-to-tool disparity (or the variation in thedimension measurements) is considered to be the magnification differenceFe between the microscopes. Therefore, the deflection coil magnificationcontrol value Pe (that is, the control parameter of the device forsetting the magnification of the image) may be adjusted to reduce thetool-to-tool disparity.

First Example

There will now be described a first exemplary tool-to-tool matchingcontrol system including scanning electron microscopes according to thefirst embodiment of the present invention capable of calibrating thesemicroscopes to reduce tool-to-tool disparities between them.

(1) System Configuration

FIG. 1 shows the configuration of the first exemplary tool-to-toolmatching control system including scanning electron microscopesaccording to the present invention. This system is made up primarily ofa plurality of scanning electron microscopes (length measuring SEMs orCDSEMs) 10 and a tool-to-tool matching control apparatus 11 forcontrolling, or correcting, tool-to-tool disparities between thesemicroscopes. The microscopes 10 and the tool-to-tool matching controlapparatus 11 are connected to each other by a data bus, or network, 12.

FIG. 2 shows an exemplary configuration of the individual scanningelectron microscopes 10 shown in FIG. 1. Each scanning electronmicroscope 10 is made up of two major portions: an electron opticalsystem 201 for capturing an electron beam image; and an informationprocessing system 202 for processing the captured image to measure atarget pattern in the image.

The electron optical system 201 primarily includes: a stage 204 on whicha sample 203 is mounted; an electron gun 206 for emitting an electronbeam 205; an aperture 207 for guiding the electron beam and reducing itsdiameter; a deflector lens 208 for deflecting the electron beam; anobjective lens 209 for adjusting the focal point of the electron beam; aretarding electrode 210; a booster 211 for pulling up the secondaryelectrons emitted from the sample 203; a secondary electron detector 212having a function to convert secondary electrons into an electricalsignal; a photomultiplier tube 213 for amplifying the intensity of adetected electrical signal to a desired level; and an A/D converter 214for converting the amplified electrical signal into a digital signal. Itshould be noted that the above components 204 and 206 to 213 arecontrolled by a controller 215.

On the other hand, the information processing system 202, which measuresthe dimensions of the imaged pattern based on the digitized secondaryelectron image data, primarily includes: an image generating unit 216for generating (optical) image data from the secondary electron imagedata; a dimension measuring unit 217 for calculating a pattern dimensionfrom the image data; a storage unit 218 for storing various data such asimage data and dimension measurements; and an input/output unit 219having a function (GUI function) to receive imaging conditions andparameters for dimension measurement from the user and to output theprocessing results. These components can exchange data with each otherthrough the data bus, or network, 12. It should be noted that the imagegenerating unit 216 and the dimension measuring unit 217 may beimplemented together by a single CPU. Further, the informationprocessing system 202 can exchange data with the other scanning electronmicroscopes 10 and the tool-to-tool matching control apparatus 11through the data bus, or network 12.

FIG. 3 shows an exemplary configuration of the tool-to-tool matchingcontrol apparatus 11. The tool-to-tool matching control apparatus 11 ismade up primarily of a processing unit 301, a storage unit 302, and aninput/output unit (or input/output means) 303. The processing unit 301includes: a tool-to-tool disparity calculating section 301 a forcalculating a difference or variation in dimension measurements (namely,CD values) obtained by the scanning electron microscopes 10 (that is, atool-to-tool disparity); an image feature value calculating section 301b for, based on image data captured by the microscopes 10, calculatingthe values of the indicators, or image features, Ca to Cg, etc.(described later) indicating states of the microscopes 10; atool-to-tool disparity causing factor analyzing section 301 c forcomparing and evaluating the dimension values measured by themicroscopes 10 and the calculated values of the indicators, or imagefeatures, Ca to Cg, etc. indicating the states of the microscopes 10 toidentify a factor(s) that has caused the tool-to-tool disparity (such asthe tool-to-tool disparity causing factors Fa to Fg); and a microscopecontrol section 301 d for changing a device control parameter(s) (suchas the device control parameters Pa to Pg, etc.) for each microscope 10based on the analysis results of tool-to-tool disparity causing factors.It should be noted that the processing unit 301 may be a CPU, and thetool-to-tool disparity calculating section 301 a, the image featurevalue calculating section 301 b, the tool-to-tool disparity causingfactor analyzing section 301 c, and the microscope control section 301 dmay be programs running on the CPU. The storage unit 302 includes: ameasured dimension storage unit 302 a for storing the dimension values(or CD values) measured by each microscope 10; a tool-to-tool disparitystorage unit 302 b for storing tool-to-tool disparitys calculated fromthe dimension values (or CD values); an image data storage unit 302 cfor storing image data, etc. obtained by each microscope 10; and animage feature value storage unit 302 d for storing values of the imagefeatures Ca to Cg, etc. calculated from image data.

(2) Method for Evaluating Tool-to-Tool Disparity Causing Factors

FIG. 4 shows an exemplary entire process sequence for identifying afactor that has caused a tool-to-tool disparity.

Referring to the entire sequence shown in FIG. 4, in order to calculatea tool-to-tool disparity, first a scanning electron microscope 10 (shownin FIG. 2) performs the following sequential steps: the control unit 215sets device control parameters (step S41); the image generating unit 216in the information processing system 202 generates secondary electronimage data for dimension measurement using a reference wafer fordimension measurement, or a reference wafer for tool-to-tool disparitymeasurement (step S42); and the dimension measuring unit 217 in theinformation processing system 202 calculates a dimension (the CD value)of a target pattern based on the secondary electron image data receivedfrom the image generating unit 216 (step S43). The pattern dimensionvalue (or CD value) calculated by the microscope 10 is sent to thetool-to-tool matching control apparatus 11 through the data bus (ornetwork) 12 and stored in the measured dimension storage unit 302 a ofthe storage unit 302 in the tool-to-tool matching control apparatus 11at step S44. Then, the same scanning electron microscope 10, whosedevice control parameters have been set at step S41 as described above,further performs the following step: the image generating unit 216 inthe information processing system 202 generates secondary electron imagedata for image feature value calculation using a reference wafer fordevice control (step S45). The generated secondary electron image datais stored, for example, in the image data storage unit 302 c of thestorage unit 302 in the tool-to-tool matching control apparatus 11.Then, at step S46, based on the generated secondary electron image data,the image feature value calculating section 301 b of the processing unit301 in the tool-to-tool matching control apparatus 11 calculates thevalues of indicators, or image features, indicating states of themicroscope 10, such as the image brightness information Ca (for example,average gray value), image contrast Cb (for example, the differencebetween the highest and lowest gray values), image noise level Cc, tiltindicator value Cd, the pitch Ce of a repeated pattern, the resolutionevaluation indicator value Cf calculated based on the image, and theaxial alignment (astigmatic adjustment) indicator value Cg calculatedbased on the image. The calculated values of these indicators, or imagefeatures, are stored in the image feature value storage unit 302 d atstep S47. It should be noted that all other scanning electronmicroscopes to be subjected to tool-to-tool matching control alsoperform the above steps (step S48).

Then, the tool-to-tool disparity calculating section 301 a calculates atool-to-tool disparity from the dimension measurements (namely, CDvalues) obtained from the microscopes 10 and stored in the measureddimension storage unit 302 a, and stores it in the tool-to-tooldisparity storage unit 302 b. Further, at step S49, the tool-to-tooldisparity causing factor analyzing section 301 c determines thecorrelation between the CD values obtained from the microscopes 10 andstored in the tool-to-tool disparity storage unit 302 b and the valuesof indicators, or image features, (indicating states of the microscopes10) obtained from the microscopes 10 and stored in the image featurevalue storage unit 302 d to estimate a factor(s) (such as Fa to Fg,etc.) that has caused the tool-to-tool disparity. Then, at step S50, themicroscope control section 301 d instructs the control unit 215 of eachmicroscope 10 to change the device control parameters Pa to Pg, etc. ofthe microscope according to the estimation results, completing thetool-to-tool matching process.

There will now be described the device control parameters set by thecontrol unit 215 at step S41 of FIG. 4. These device control parametersare used to control the scanning electron microscopes 10 and 18 shown inFIGS. 2 and 8, respectively, and include: a position control parameterfor controlling the position of the stage 204; a voltage/current controlparameter for causing the electron gun 206 to emit the electron beam205; an astigmatic adjustment coil control parameter for shaping theelectron beam 205; a deflector lens 208 control parameter for settingthe amount of deflection, the scan angle, and the scan range of theelectron beam; an objective lens 209 control parameter for controllingthe focal point of the electron beam; a retarding electrode 210voltage/current control parameter for adjusting the acceleration voltageof the electron beam near the wafer; a booster 211 voltage/currentcontrol parameter for “pulling up” the emitted secondary electrons; adetection sensitivity adjustment parameter for the secondary electrondetector 212; a gain adjustment parameter for the photomultiplier tube213; and a conversion gain/offset adjustment parameter for the A/Dconverter 214. In addition, the device control parameters set by thecontrol unit 215 further include a vacuum adjustment parameter for thevacuum chamber within the microscope, a temperature/humidity adjustmentparameter for the atmosphere around the electron beam path, and atemperature/humidity parameter for the atmosphere around the microscope.

There will now be specifically described step S42 in which eachmicroscope 10 captures secondary electron image data for CD valuecalculation.

The pattern to be measured on the reference wafer for dimensionmeasurement is preferably configured such that even a slight change inthe state of the microscope leads to a significant change in the CDvalue (obtained by the microscope). Such patterns include those having asharp edge. Further, the pattern dimensions (critical dimension)preferably do not vary much across the surface of the wafer. The reasonfor this is that when the CD values measured by microscopes are comparedwith each other, each CD value should not depend on the position on thepattern where it is measured. Actually, each microscope measures thecritical dimension (CD) at a plurality of positions, and the average ofthese CD values is used as a representative CD value for comparison. Ifthe pattern dimensions (critical dimension) vary widely across the wafersurface, the critical dimension (CD) of the pattern must be measured ata larger number of positions in order to eliminate the dependence of theaveraged (or representative) CD value on the position on the pattern andthereby ensure sufficient measurement accuracy.

The secondary electron image data capturing step (S42) using such areference wafer for dimension measurement will now be specificallydescribed with reference to FIG. 5. First of all, a reference wafer 203for dimension measurement having a pattern to be measured thereon isplaced on the stage 204 at step S421. Then, at step S422, in response toan instruction from the control unit 215, the stage 204 is moved to aposition at which an image of the pattern to be measured can be capturedthrough electron beam irradiation. After that, the electron beam 205 isemitted from the electron gun 206, passed through the aperture 207, anddeflected by the primary electron beam deflector 208 so as to scan theelectron beam across the surface of the sample (or wafer) on the stageat step S423. At that time, the objective lens 209 is controlled so asto capture an focused image, and furthermore the retarding electrode 210is activated so that the captured image has a high resolution.

Then, the secondary electrons (or signal) emitted from the samplescanned with the electron beam are “pulled up” to the secondary electrondetector 212 by the booster 211, so that the detector detects thesecondary electrons (or signal) at step S424. At that time, a value setfor the objective lens 209 (that is, the objective lens control valuewhen the image was captured) is read from the control unit 215 andstored in the storage unit 218 as the magnification of the capturedimage. The reason for storing this value is that, since themagnification of the captured image varies slightly depending on thesettings of the objective lens 209, this slight change in themagnification must be reflected in the later CD value calculationprocessing in order to calculate an accurate CD value. Lastly, at stepS425, the captured secondary electron signal is amplified by thephotomultiplier tube 213 and then converted into a digital signal(digitized secondary electron data) by the A/D converter 214, and thensecondary electron image data is generated from the digitized secondaryelectron data by the image generating unit 216.

The generated secondary electron image data is stored in the storageunit 218 of the scanning electron microscope 10 and in the image datastorage unit 302 c of the tool-to-tool matching control apparatus 11.All other scanning electron microscopes to be subjected to tool-to-toolmatching control also perform the above steps, thus capturing secondaryelectron image data at a number of locations on the pattern to bemeasured.

With reference to FIG. 6, there will now be specifically described stepS43 of FIG. 4 in which the dimension measuring unit 217 of eachmicroscope 10 calculates a CD value from secondary electron image datain order to evaluate tool-to-tool disparity causing factors. First, animage profile is produced from secondary electron image data.Specifically, at step S431, an image area 72 for preparing a profile isselected from a captured microscopic image 71 shown in FIG. 7A. Theimage area 72 must be such that: it contains at least the entire patterndimension to be measured (hereinafter referred to as “the dimension ofthe pattern in the X-direction”); and there are as many pixels in theY-direction (that is, a direction perpendicular to the pattern dimensionto be measured) as necessary to calculate an appropriate average pixelvalue and thereby calculate an accurate dimension value (in theX-direction). Specifically, the image area must includes a few hundredsof pixels in the Y-direction to reduce the noise inherent in scanningelectron microscopes. The more pixels used to calculate an average pixelvalue, the larger the reduction in the noise components inherent in thescanning electron microscope. It should be noted that the dimensions ofthe image area in the X- and Y-directions may be changed as necessarydepending on the shape of the pattern to be measured.

Then, in the selected image area 72, the values of the pixels in eachcolumn in the Y-direction are averaged to produce a single-line waveform73 as shown in FIG. 7B at step S432. Lastly, at step S434, thesingle-line waveform is smoothed by a filter having a filter parameterspecified at step S433, producing an image profile 74 as shown in FIG.7C. It should be noted that the above filter processing (step S434) maybe omitted.

Then, a CD value is calculated from the generated image profile 74 atstep S435. There are various method for calculating the CD value.According to the present embodiment, for example, the distance in theX-direction between the middle points of both slanted outside lines ofthe image profile is set as the CD value (75), as shown in FIG. 7C.

This completes the detailed description of the step (S43) of calculatinga CD value based on secondary electron image data.

Actually, CD values are calculated from the secondary electron imagedata captured at a number of locations on the pattern at step S42 ofFIG. 4, and the average or median of these calculated CD values is setas a representative CD value for the microscope. (The distribution rangeof the calculated CD values is assumed to constitute the error range.)This process of calculating CD values and averaging them is performedfor each scanning electron microscope. Each calculated (representative)CD value is stored in the storage unit 218 of a respective scanningelectron microscope 10 and in the measured dimension storage unit 302 aof the tool-to-tool matching control apparatus 11.

There will now be specifically described step S45 of FIG. 4 at whicheach microscope 10 captures secondary electron image data forcalculation of values of indicators, or image features, indicatingstates of the microscope.

An image feature (as used herein) is a feature in an image that reflectsa state of the microscope that captured the image. Examples of imagefeatures include image brightness information Ca (for example, anaverage brightness level), image contrast Cb, image noise level Cc, tiltindicator value Cd, the pitch Ce of a repeated pattern, resolutionevaluation indicator value Cf calculated based on the image, and axialalignment (astigmatic adjustment) indicator value Cg also calculatedbased on the image.

Of these image features, the average image brightness level Ca and theimage contrast Cb can be determined based on a histogram of gray valuesin the image, and the image noise level Cc is determined based on thevariation in the (light) intensity of a portion of the image having nopattern image. Further, the tilt indicator value Cd can be determined bycapturing secondary electron image data of a pyramidal pattern for tiltcalculation and analyzing the edge lines of the pyramid. Further, thepitch Ce of a repeated pattern can be measured from secondary electronimage data of the repeated pattern. Still further, the resolutionevaluation indicator value Cf can be determined by several knownmethods. For example, the indicator value Cf may be determined byapplying DFT (Discrete Fourier Transform) to the image or calculatingautocorrelation function (ACF) coefficients. Or it may be determined bymeasuring the width of convex portions of the image (referred to as theABW method). Specifically, for example, a method using DFT calculatesthe shortest period in the original image from the DFT of the image andsets it as the resolution evaluation indicator value. Further, theastigmatic adjustment indicator value Cg can be determined bycalculating resolution evaluation indicator values in a plurality ofdirections and thereby obtaining the distortion of the shape of the beamin each direction. The values of each indicator obtained by themicroscopes are compared with each other to identify a factor(s) thathas caused the tool-to-tool disparity. Such a factor(s) may be the imagebrightness difference Fa, image contrast difference Fb, image noiselevel difference Fc, relative tilt angle Fd, image magnificationdifference Fe, resolution difference Ff, or axial alignment (astigmaticadjustment) accuracy difference Fg. The calculated values of the imagefeatures are stored in the image feature value storage unit 302 d of thetool-to-tool matching control apparatus 11.

In order to calculate the values of these image features, the referencewafer for device (microscope) control has thereon a pyramidal patternfor tilt evaluation, a repeated pattern for pattern pitch measurement,and a dense pattern having edges in all radial directions forcalculation of resolution evaluation indicator values and an astigmaticadjustment indicator value, in addition to line, hole, and dot patterns.It should be noted that it is possible to use only a single referencewafer having the functions of both the reference wafer for devicecontrol and the reference wafer for dimension measurement.

Secondary electron image data of the above reference wafer for devicecontrol is captured in the same manner as described with reference toFIG. 5. The values of each image feature obtained by microscopespreferably do not have any dependence on the position on the patternwhere they are measured so that they can be properly compared with eachother, as in the case of calculation of CD values. Therefore, actually,each microscope measures the same image feature at a number of positionson the pattern using many captured secondary electron images, and theaverage of the measured values of each image feature is used as arepresentative value of the feature for the microscope. The capturedsecondary electron image data is stored in the storage unit 218 of thescanning electron microscope 10 and in the image data storage unit 302 cof the tool-to-tool matching control apparatus 11.

There will now be specifically described step S46 of FIG. 4 at which theimage feature value calculating section 301 b of the tool-to-toolmatching control apparatus 11 calculates values of various imagefeatures. As in the case of calculation of CD values, the image featurevalue calculating section 301 b reads the secondary electron image data(captured at a number of locations on the pattern by each microscope)from the image data storage unit 302 c, calculates values of each imagefeature from the image data, and sets the average or median of thesevalues as a representative value of the feature for the microscope. (Thedistribution range of the calculated values is assumed to constitute theerror range.) Thus, the image feature value calculating section 301 bcalculates a representative value of each image feature (Ca to Cg, etc.)for each microscope. Examples of how to determine the value of eachimage feature (Ca to Cg, etc.) were described above. The calculated(representative) value of each image feature (Ca to Cg, etc.) is storedin the storage unit 218 of the scanning electron microscope 10 and inthe image feature value storage unit 302 d of the tool-to-tool matchingcontrol apparatus 11.

The tool-to-tool disparity causing factor analyzing section 301 cdetermines the correlation between the dimension values (or CD values)obtained from the microscopes 10 and stored in the tool-to-tooldisparity storage unit 302 b and the values of each indicator, or imagefeature, Ca to Cg, etc. (indicating a state of each microscope 10)obtained from the microscopes 10 and stored in the image feature valuestorage unit 302 d, and thereby estimates a factor that has caused thetool-to-tool disparity. Such a factor(s) may be the image brightnessdifference Fa, image contrast difference Fb, image noise leveldifference Fc, relative tilt angle Fd, image magnification differenceFe, resolution difference Ff, or the axial alignment accuracy differenceFg.

There will now be specifically described step S49 of FIG. 4 at which thetool-to-tool-variation causing factor analyzing section 301 c of thetool-to-tool matching control apparatus 11 estimates a factor(s) thathas caused the tool-to-tool disparity. First, the tool-to-tool disparitycausing factor analyzing section 301 c determines the correlationbetween the CD values obtained from the microscopes 10 and stored in themeasured dimension storage unit 302 a and the values of each imagefeature calculated at step S46 of FIG. 4 and stored in the image featurevalue storage unit 302 d. If these CD values and values of an imagefeature have a higher correlation than a predetermined threshold value,the tool-to-tool disparity causing factor analyzing section 301 cdetermines that the tool-to-tool disparity causing factor (Fa to Fg, oretc.) indicated by this image feature has caused the tool-to-tooldisparity. It should be noted that principal component analysis or othertechnique for evaluating the dependence of CD values on an image featuremay be used instead of correlation evaluation between CD values and animage feature.

There will now be specifically described step S50 of FIG. 4 at which themicroscope control unit 301 d instructs each microscope 10 to change itsdevice control parameters. If the CD values have a high correlation withone of the indicators, or image features, Ca to Cg, etc. (indicatingstates of the microscopes), the device control parameter (Pa to Pg, oretc.) corresponding to the tool-to-tool disparity causing factor (Fa toFg, or etc.) indicated by the correlated indicator is adjusted. Thecorresponding relationship between the tool-to-tool disparity causingfactors (Fa to Fg) and the device control parameters (Pa to Pg) to beadjusted is as follows: the image brightness (or average brightnesslevel) difference Fa and the image contrast difference Fb (that is, thedifference in the (macroscopic) brightness level distribution) can beadjusted by changing the photomultiplier gain Pa or the probe current(or electron beam current) Pb; the image noise level difference Fc, theprobe current (or electron beam current) Pc; the relative tilt angle Fd,the beam tilt control value of the deflection coil or the stageinstallation angle, Pd; the image magnification difference Fe, thedeflection coil magnification control value Pe; the resolutiondifference Ff, the objective lens control value, retarding controlvalue, or the aperture (changed by replacing the mechanism) Pf; and theastigmatic adjustment accuracy difference Fg, the axial alignment coil(astigmatic adjustment coil) control value Pg.

The required amount of adjustment of each device control parameter (Pato Pg, etc.) may be determined as follows. (1) When a device controlparameter needs to be adjusted, the required amount of adjustment isdetermined so as to minimize the value of the corresponding imagefeature or the tool-to-tool disparity. Alternatively, (2) therelationship between the amount of adjustment of each device controlparameter (Pa to Pg, etc.) and the amount of change in the value of eachimage feature (Ca to Cg, etc.) is found beforehand, and the requiredamount of adjustment of each device control parameter is determinedbased on this relationship.

Information about the required amount of adjustment of each devicecontrol parameter (Pa to Pg, etc.) is sent from the microscope controlsection 301 d of the tool-to-tool matching control apparatus 11 to thecontrol unit 215 of a respective microscope which then adjusts itsdevice control parameters according to this information.

There will now be described exemplary input/output screens displayed bythe input/output unit 219 of the information processing system 202 ineach microscope and by the input/output unit 303 of the tool-to-toolmatching control apparatus 11.

FIG. 8 shows an exemplary icon (or button) displayed by the input/outputunit 219 of each scanning electron microscope 10, wherein the button ispressed by the user to obtain data for evaluation of tool-to-tooldisparity causing factors. FIG. 9 shows an exemplary screen with anexemplary icon (or button) thereon displayed by the input/output unit303 of the tool-to-tool matching control apparatus 11 before evaluationof tool-to-tool disparity causing factors, wherein: the screen is usedto specify a scanning electron microscope(s) and data (device controlparameter data of the microscope(s), etc.) for evaluation oftool-to-tool disparity causing factors; and the button is pressed by theuser to start the evaluation.

FIG. 10 includes exemplary output screens (1000) displayed by thissystem, showing evaluation results of tool-to-tool disparity causingfactors (produced by the tool-to-tool-disparity causing factor analyzingsection 301 c). Referring to the figure, reference numeral 1001 denotesa graph showing the relationship between dimension values or CD values(indicated by a circle) measured by microscopes A to Z and the values ofan image feature 1 (indicated by a rhombus), values of an image feature2 (indicated by a triangle), . . . , and values of an image feature N(indicated by a square) in images captured by these microscopes A to Z.It should be noted that this graph is also considered to indicatedifferences (or the tool-to-tool disparities) between a CD valuemeasured by a reference microscope and CD values measured by themicroscopes A to Z and the differences between the values of these imagefutures 1 to N in an image captured by the reference microscope and thevalues of the image futures 1 to N in images captured by microscopes Ato Z. Further, reference numeral 1002 denotes a table showing the samerelationship as graph 1001. Further, reference numeral 1003 denotes agraph showing the correlation values of the image features 1 to N withthe dimension values (or CD values) measured by the microscopes A to Z.(The differences between these CD values constitute tool-to-tooldisparities between the microscopes A to Z.) It should be noted thatthis graph is also considered to indicate the correlation values of theimage features 1 to N with the differences (or tool-to-tool disparities)between a CD value measured by a reference microscope and CD valuesmeasured by the microscopes A to Z. Further, reference numeral 1004denotes a table showing the same relationship as graph 1003. As shown ingraph 1003 and table 1004, among these image features, the image feature2 has the highest correlation with the tool-to-tool disparity or CDvalues (indicated by a circle). Therefore, the factor indicated by theimage feature 2 is determined to have caused the tool-to-tool disparity.In this way, each (candidate) tool-to-tool disparity causing factor canbe examined.

Further, FIG. 10 also includes exemplary input/output screens forautomatically adjusting the scanning electron microscopes based oninformation about (candidate) tool-to-tool disparity causing factors soas to reduce the tool-to-tool disparity. Referring to the figure,reference numeral 1005 denotes a table showing the relationship betweenthe image features 1 to N and the candidate device control parameters(Pa to Pg, etc.) that may be adjusted when the factors (Fa to Fg, etc.)indicated by these image features 1 to N have caused a tool-to-tooldisparity. This relationship is stored in the storage unit 302 by themicroscope control section 301 d as a database beforehand. Further,reference numeral 1006 denotes a table showing, for example, changes in(or histories of) the values of some device control parameters of themicroscope A. Further, reference numeral 1008 denotes an icon (orbutton) for changing device control parameters. This button may or maynot be selected depending on the output results. Thus, the input/outputunit (input/output means) 303 displays the above input/output screens(1000), and the microscope control section 301 d can select anappropriate device control parameter(s) for a factor that has caused thetool-to-tool disparity and adjust the selected device control parameterby an appropriate amount to eliminate the factor so as to correct orreduce the tool-to-tool disparity.

That is, the storage unit 302 stores a database storing the relationshipbetween tool-to-tool disparity causing factors (Fa to Fg, etc.) anddevice control parameters (Pa to Pg, etc.) for adjusting thesetool-to-tool disparity causing factors, and based on this relationship,the microscope control section 301 d selects an appropriate devicecontrol parameter(s) for the tool-to-tool disparity causing factordisplayed and output by the input/output unit 303 and instructs thecontrol unit 215 of each microscope to automatically adjust the selecteddevice control parameter(s) for the microscope by an appropriate amountto reduce the tool-to-tool disparity between the microscopes.

According to the present embodiment described above, it is possible tocorrect or reduce tool-to-tool disparities between scanning electronmicroscopes.

Example 2

A second exemplary tool-to-tool matching control system of the presentinvention (according to the first embodiment) will now be described.This tool-to-tool matching control system is different from that ofExample 1 in that, instead of the tool-to-tool matching controlapparatus 11, the information processing system 202 in each scanningelectron microscope 10 a includes an image feature value calculatingsection 301 b, as shown in FIG. 11.

Therefore, in each scanning electron microscope 10 a, the image featurevalue calculating section 301 b reads secondary electron image data fromthe storage unit 218 and calculates the value of each image feature inthe secondary electron image data. The calculated value of each imagefeature is sent to a tool-to-tool matching control apparatus 15 throughthe data bus, or network, 12 and stored in the image feature valuestorage unit 302 d of the tool-to-tool matching control apparatus 15.

This example allows scanning electron microscopes to correct or reducetool-to-tool disparities between them.

Example 3

A third exemplary tool-to-tool matching control system of the presentinvention (according to the first embodiment) will now be described.This tool-to-tool matching control system is different from those ofExamples 1 and 2 in that this system does not include a separatetool-to-tool matching control apparatus 11 such as that of Examples 1and 2, but the scanning electron microscopes 19 c each includes atool-to-tool matching control apparatus 11 instead, as shown in FIG. 12.(These tool-to-tool matching control apparatuses 11 are connected toeach other through the data bus, or network, 12.) As a result, accordingto Example 3, each scanning electron microscope 10 c is made upprimarily of: an electron optical system 201 for capturing an electronbeam image; an information processing system 202 for processing thecaptured image to measure the dimensions of a target pattern in theimage; and a tool-to-tool-disparity causing factor evaluating unit 11for controlling and correcting tool-to-tool disparities.

Thus, the scanning electron microscope system of Example 3 includes aplurality of scanning electron microscopes each having a tool-to-tooldisparity correcting function that are connected to each other throughthe data bus, or network, 12, as shown in FIG. 13.

This example also allows scanning electron microscopes to correct orreduce tool-to-tool disparities between them.

Example 4

A fourth exemplary tool-to-tool matching control system of the presentinvention (according to the first embodiment) will now be described.This tool-to-tool matching control system is different from those ofExamples 1 to 3 in that the tool-to-tool matching control apparatus (11a) described in connection with Example 1 is not connected to thescanning electron microscopes, as shown in FIG. 14.

However, according to Example 4, secondary electron image data and CDvalues can be supplied from each microscope to the tool-to-tool matchingcontrol apparatus 11 a through its input/output unit 303, and theprocessing unit 301 and the storage unit 302 in the tool-to-toolmatching control apparatus 11 a allow outputting an instruction tochange a device control parameter(s) to a new value through theinput/output unit 303. The processing within the tool-to-tool matchingcontrol apparatus 11 a is the same as described in connection withExample 1.

FIGS. 9 and 15 show exemplary input screens used by the tool-to-toolmatching control system of Example 4. Specifically, FIG. 15 shows anexemplary input screen 1500 used when entering dimension values (or CDvalues) and secondary electron image data of a target pattern obtainedby a plurality of scanning electron microscopes. This screen is used tospecify the data storage method, or data storage locations, and includesan input portion (or button) 1502 for entering the input data paths(1501) and information on the input data. FIG. 9 shows an exemplaryscreen with an exemplary icon (or button) thereon, wherein: the screenis used to specify a scanning electron microscope(s) and data forevaluation of tool-to-tool disparity causing factors; and the button ispressed by the user to start the evaluation.

This example also allows scanning electron microscopes to correct orreduce tool-to-tool disparities between them based on information ontoll-to-tool disparity causing factors.

Second Embodiment

It is not easy to pinpoint a factor that has caused a variation indimension measurements between measuring scanning electron microscopes,since they have a sophisticated configuration.

A second embodiment of the present invention has been devised to addressthis problem. The second embodiment is different from the firstembodiment in that instead of using features in images captured by eachscanning electron microscope, the second embodiment uses various devicestate parameters of each microscope to evaluate tool-to-tool disparitycausing factors. Such device (or microscope) state parameters includeactual control parameters, environmental indicators (such as the vacuumof the sample chamber), etc.

That is, a tool-to-tool matching control system of the presentembodiment automatically monitors various device state parameters ofeach microscope, such as actual control parameters and environmentalindicators (the vacuum of the sample chamber, etc.), in order toevaluate tool-to-tool disparity causing factors. The dimension valuesmeasured by the microscopes are compared with the monitored values ofeach device state parameter to evaluate tool-to-tool disparity causingfactors and thereby identify a factor(s) that has caused thetool-to-tool disparity and automatically calibrate each microscopeaccording to the evaluation results. For example, when calculating atool-to-tool disparity between microscopes, the values of each devicestate parameter exhibited by the microscopes are measured, and thecorrelation between these values and the CD values measured by themicroscopes is determined. Change in the device state parameter(s)having high correlation with the CD values is considered to be a factorthat has caused the tool-to-tool disparity. Therefore, the tool-to-tooldisparity can be reduced by adjusting this device state parameter. Theactual control parameters of each microscope described above areconsidered to vary due to component variation or age. Therefore, it isimportant to examine these control parameters of each microscope.(Changing the value of the same control parameter of each microscope byan equal amount may not lead to a desired result.)

Example 5

There will now be described a fifth exemplary tool-to-tool matchingcontrol system including scanning electron microscopes according to thesecond embodiment, capable of calibrating these microscopes so as toreduce tool-to-tool disparities between them.

(1) System configuration

FIG. 16 shows the configuration of a fifth exemplary tool-to-toolmatching control system including scanning electron microscopesaccording to the present invention. This system is made up primarily ofa plurality of scanning electron microscopes 18 (length measuring SEMsor CDSEMs) and a tool-to-tool matching control apparatus 19 forcontrolling tool-to-tool disparities between these microscopes. Themicroscopes 18 and the tool-to-tool matching control apparatus 19 areconnected to each other through a data bus, or network, 12.

FIG. 17 shows an exemplary configuration of the individual scanningelectron microscopes 18. Each scanning electron microscope 18 is made upof two major portions: an electron optical system 1901 for capturing anelectron beam image; and an information processing system 202 forprocessing the captured image to measure a target pattern in the image.

The electron optical system 1901 is different from the electron opticalsystem 201 shown in FIG. 2 in that it includes a measuring unit 230 formeasuring or monitoring various device state parameters (correspondingto input device control parameters) of the components in eachmicroscope, such as the electron gun voltage/current A, deflector lenscurrent B, objective lens current C, retarding current/voltage D,booster current/voltage E, input/output values F of the secondaryelectron detector, and input/output values G of the photomultipliertube, and various environmental indicators (described below). Examplesof environmental indicators include, for example, the temperature,humidity, degree of vacuum, electric field, magnetic field, andvariation at each location in the microscope, the operating time of themicroscope, and the charge state of the sample.

The configuration of the information processing system 202, on the otherhand, is the same as that shown in FIG. 2. It should be noted that thestorage unit 218 stores the values of various device state parametersmeasured by the measuring unit 230 when tool-to-tool disparity causingfactors are evaluated. Further, the information processing system 202can exchange data with other scanning electron microscopes 18 and thetool-to-tool matching control apparatus 19 through a data bus, ornetwork, 12.

Further, for example, the tool-to-tool matching control apparatus 19 ismade up primarily of a processing unit 301′, a storage unit 302, and aninput/output unit 303, as shown in FIG. 18. The processing unit 301′includes: a tool-to-tool disparity calculating section 301 a forcalculating a difference or variation in dimension measurements (namely,CD values) obtained by the scanning microscopes 18 (that is, atool-to-tool disparity); a tool-to-tool-disparity causing factoranalyzing section 301 e for comparing and evaluating the dimensionvalues (or CD values) measured by the microscopes 18 and the values ofvarious device state parameters exhibited by the microscopes 18 toidentify a factor that has caused the tool-to-tool disparity; and amicroscope control section 301 d for changing device control parametersof each microscope 18 based on the analysis results of tool-to-tooldisparity causing factors. The storage unit 302′ includes: a measureddimension storage unit 302 a for storing the dimension values (or CDvalues) measured by the microscopes 18; a tool-to-tool disparity storageunit 302 b for storing tool-to-tool disparities calculated from thedimension values; and a device state parameter storage unit 302 e forstoring values of various device state parameters.

(2) Method for Evaluating Tool-to-Tool Disparity Causing Factors

FIG. 19 shows an exemplary entire process sequence for identifying afactor(s) that has caused a tool-to-tool disparity.

This process sequence is different from that shown in FIG. 4 in that itincludes the following steps: the measuring unit 230 of each scanningelectron microscope 18 measures the value of each device state parameterexhibited by the microscope when a CD value of the pattern to bemeasured is calculated (step S51); the storage unit 218 of eachmicroscope 18 and the device state parameter storage unit 302 e storethe measured values of the device state parameters (step S52); and thetool-to-tool disparity causing factor analyzing section 301 e estimatesa factor(s) that has caused the tool-to-tool disparity based on therelationship between the CD values measured by the microscopes 18 andthe values of each device state parameter exhibited by these microscopes(step S53). In this process sequence, after step S53, at step S50 themicroscope control section 301 d instructs the control unit 215 of eachmicroscope 18 to change device control parameters for the microscopeaccording to the estimation results, completing the tool-to-tool machingprocess. It should be noted that steps S41 to S44, S48, and S50 in thesequence of FIG. 19 are the same as those described with reference withFIG. 4. It should be further noted that the device control parametersset for each microscope at step S41 are the same as described above.

There will now be specifically described various device state parametersmeasured at step 51 of FIG. 19. Examples of device state parametersmeasured by the measuring unit 230 in each scanning electron microscope18 include: parameters that affect the state of the electron beam, suchas the electron gun voltage/current A, deflection lens current B,objective lens current C, retarding current/voltage D, and boostercurrent/voltage E; parameters that affect the state of the secondaryelectrons emitted from the target pattern, such as the input/outputvalues F of the secondary electron detector and input/output values G ofthe photomultiplier tube; and parameters that may cause an error in thecaptured secondary electron image data, such as the stage stoppingaccuracy H, the temperature I, humidity J, vibration K, magnetic fieldL, electric field M, and degree of vacuum N (in the vacuum chamber) ateach location in the microscope, the operating time O of the microscope,and the charge state P of the sample. The measured values of thesedevice state parameters are stored in the device state parameter storageunit 302 e of the tool-to-tool matching control apparatus 19.

There will now be specifically described step S53 of FIG. 19 thatestimates a factor(s) that has caused a tool-to-tool disparity. Thisstep is performed by the tool-to-tool-disparity causing factor analyzingsection 301 e in the tool-to-tool disparity control apparatus 19. Thetool-to-tool-disparity causing factor analyzing section 301 e determinesthe correlation between the CD values measured by the microscopes 18 andstored in the measured dimension storage unit 302 a of the tool-to-toolmatching control apparatus 19 and the values of each device stateparameter measured at step S51 of FIG. 19 and stored in the device stateparameter storage unit 302 e. If these measured CD values and values ofa device state parameter have a higher correlation than a thresholdvalue, the tool-to-tool-disparity causing factor analyzing section 301 edetermines that the tool-to-tool disparity causing factor indicated bythis device state parameter has caused the tool-to-tool disparity.

It should be noted that principal component analysis or other techniquefor evaluating the dependence of CD values on a device state parametermay be used instead of correlation evaluation between CD values and adevice state parameter.

There will now be described step S50 of FIG. 19 that changes devicecontrol parameters. It should be noted that since the device stateparameters A to H have the corresponding device control parameters,these corresponding device control parameters may be set so as to setthe device state parameters A to H to desired values. On the other hand,the environmental indicators I to P are preferably automaticallyadjusted to desired values. However, if such an arrangement is difficultto implement, the input/output unit 303 of the tool-to-tool matchingcontrol apparatus 19 may send an alarm to notify the user of thesituation. Further, the required amount of adjustment of each devicecontrol parameter may be determined as follows. (1) When a devicecontrol parameter needs to be adjusted, the required amount ofadjustment is determined so as to minimize the value of thecorresponding device state parameter or the tool-to-tool disparity.Alternatively, (2) the relationship between the amount of adjustment ofeach device control parameter and the amount of change in each devicestate parameter is found beforehand, and the required amount ofadjustment of each device control parameter is determined based on thisrelationship. Information about the required amount of adjustment ofeach device control parameter is sent from the microscope controlsection 301 d of the tool-to-tool matching control apparatus 19 to thecontrol unit 215 of each microscope which then adjusts its devicecontrol parameters according to this information.

An exemplary input screen employed by this system will be described.

FIG. 8 shows an exemplary icon (or button) displayed by each scanningelectron microscope 18, wherein the button is pressed by the user toobtain data for evaluation of tool-to-tool disparity causing factors.FIG. 9 shows an exemplary screen with an exemplary icon (or button)thereon displayed by the tool-to-tool matching control apparatus 19before evaluation of tool-to-tool disparity causing factors, wherein:the screen is used to specify a scanning electron microscope(s) and datafor evaluation of tool-to-tool disparity causing factors; and the buttonis pressed by the user to start the evaluation.

Further, FIG. 20 includes exemplary output screens (2000) displayed bythis system. Referring to the figure, reference numeral 2001 denotes agraph showing the relationship between dimension values or CD values(indicated by a circle) measured by microscopes A to Z and the values ofa device state parameter 1 (indicated by a rhombus), the values of adevice state parameter 2 (indicated by a triangle), . . . , and thevalues of a device state parameter N (indicated by a square) exhibitedby the microscopes A to Z. It should be noted that this graph is alsoconsidered to indicate the differences (or tool-to-tool disparities)between a CD value measured by a reference microscope and CD valuesmeasured by the microscopes A to Z and the differences between thevalues of the device state parameters 1 to N exhibited by the referencemicroscope and the values of the device state parameters 1 to Nexhibited by the microscopes A to Z. Further, reference numeral 2002denotes a table showing the same relationship as graph 2001.

Further, reference numeral 2003 denotes a graph showing the correlationvalues of the device state parameters 1 to N with the dimension values(or CD values) measured by the microscopes A to Z. (The differencesbetween these CD values constitute the tool-to-tool disparities betweenthe microscopes A to Z.) It should be noted that this graph is alsoconsidered to indicate the correlation values of the device stateparameters 1 to N with the differences (or tool-to-tool disparities)between a CD value measured by a reference microscope and CD valuesmeasured by the microscopes A to Z. Further, reference numeral 2004denotes a table showing the same relationship as graph 2003. As shown ingraph 2003 and table 2004, the device state parameter 2 has the highestcorrelation with the tool-to-tool disparity (or CD values). Therefore,change in the factor indicated by the device state parameter 2 isdetermined to have caused the tool-to-tool disparity. In this way, each(candidate) tool-to-tool disparity causing factor can be examined.

Further, FIG. 20 also includes exemplary input/output screens forautomatically adjusting the scanning electron microscopes based oninformation about (candidate) tool-to-tool disparity causing factors soas to reduce the tool-to-tool disparity. Referring to the figure,reference numeral 2005 denotes a table showing the relationship betweenthe device state parameters 1 to N and the candidate device control (oradjustment) parameters that may be adjusted when the factors indicatedby these device state parameters have caused a tool-to-tool disparity.This relationship is stored in the storage unit 302 by the microscopecontrol section 301 d as a database beforehand. Further, referencenumeral 2006 denotes a table showing, for example, changes in (orhistories of) the values of some device control parameters of themicroscope A. Further, reference numeral 2008 denotes an icon (orbutton) for changing device control parameters. (This button may or maynot be selected depending on the output results.) Thus, the input/outputunit 303 displays the input/output screens (2000), and the microscopecontrol section 301 d can select an appropriate device controlparameter(s) for a factor that has caused the tool-to-tool disparity andadjust the selected device control parameter by an appropriate amount toeliminate the factor so as to correct or reduce the tool-to-tooldisparity.

That is, the storage unit 302 stores a database storing the relationshipbetween tool-to-tool disparity causing factors and device controlparameters for adjusting these tool-to-tool disparity causing factors,and based on this relationship, the microscope control section 301 dselects an appropriate device control parameter(s) for the tool-to-tooldisparity causing factor displayed and output by the input/output unit303 and instructs the control unit 215 of each microscope toautomatically adjust the selected device control parameter(s) for themicroscope by an appropriate amount to reduce the tool-to-tool disparitybetween the microscopes.

According to the present embodiment described above, it is possible tocorrect or reduce tool-to-tool disparities between microscopes.

Example 6

A sixth exemplary tool-to-tool matching control system of the presentinvention (according to the second embodiment) is different from that ofExample 5 in that this system does not include a separate tool-to-toolmatching control system 19 such as that of Example 5, but the scanningelectron microscopes each includes a tool-to-tool matching controlapparatus 19 instead, as shown in FIG. 12.

Example 7

A seventh exemplary tool-to-tool matching control system of the presentinvention (according to the second embodiment) is different from that ofExample 5 in that the tool-to-tool matching control apparatus 19 is notconnected to the scanning electron microscopes through the data bus (ornetwork) 12, as shown in FIG. 14.

However, according to Example 7, device state parameter values and CDvalues can be supplied from each microscope to the tool-to-tool matchingcontrol apparatus 19 through its input/output unit 303, and thetool-to-tool matching control apparatus 19 can output an instruction tochange a device control parameter(s) to a new value through theinput/output unit 303. The processing within the tool-to-tool matchingcontrol apparatus 19 is the same as described in connection with Example5.

FIGS. 9 and 21 show exemplary input screens used by the tool-to-toolmatching control system of Example 7. Specifically, FIG. 21 shows anexemplary input screen 2100 used when entering the dimension values (orCD values) of a target pattern measured by a plurality of scanningelectron microscopes and values of each device state parameter exhibitedby the microscopes. This screen is used to specify a data storagemethod, or data storage locations, and includes an input portion (orbutton) 2102 for entering the input data paths (2101) and informationabout the input data. FIG. 9 shows an exemplary screen with an exemplaryicon (or button) thereon, wherein: the screen is used to specify ascanning electron microscope(s) and data for evaluation of tool-to-tooldisparity causing factors; and the button is pressed by the user tostart the evaluation.

This example also allows scanning electron microscopes to correct orreduce tool-to-tool disparities between them based on information ontool-to-tool disparity causing factors.

Third Embodiment

The first embodiment analyzes tool-to-tool disparity causing factorsbased on image features, and the second embodiment does such analysisusing device state parameters, as described above. A third embodiment ofthe present invention, on the other hand, analyzes tool-to-tooldisparity causing factors based on both image features and device stateparameters.

Fourth Embodiment

The first to third embodiments of the present invention provide scanningelectron microscope systems that can correct or reduce tool-to-tooldisparities between the microscopes. The fourth embodiment of thepresent invention provides a variation of these scanning electronmicroscope systems which can estimate a factor(s) that has caused avariation in dimension measurements obtained by the same microscope at aseries of times, that is, over time, and reduce such a variation. Inthis case, the horizontal axes of graphs 1001 and 2001 in FIGS. 10 and20, respectively, must be changed so as to represent time. (Originallymicroscope reference numbers (A to Z) are plotted on these axes.) Itshould be noted that the scanning electron microscope system of thefourth embodiment is made up of at least one scanning electronmicroscope (10 or 18) and one tool-to-tool matching control apparatus(11 or 19).

Fifth Embodiment

The scanning electron microscope systems of first to fourth embodimentof the present invention can estimate a factor(s) that has caused avariation (or tool-to-tool disparity) in dimension measurements betweenmeasuring microscopes or variation in dimension measurements over timeand automatically calibrate the microscopes based on the estimationresults. On the other hand, a scanning electron microscope system of afifth embodiment of the present invention does not automaticallycalibrate the microscopes. According to the fifth embodiment, the usermay calibrate the microscopes based on the following information(displayed by the input/output unit 219 or 303 of the system): thetool-to-tool disparity or the variation in dimension measurements overtime; the results of estimating a factor that has caused thetool-to-tool disparity or the variation; and the candidate devicecontrol parameter(s) to be adjusted and the required amount ofadjustment determined based on the estimation results.

Sixth Embodiment

The tool-to-too matching control systems (or scanning electronmicroscopes systems) of the first to fifth embodiments of the presentinvention estimate a factor(s) that has caused a tool-to-tool disparityeach time they measure the disparity. According to a sixth embodiment ofthe present invention, the frequency of this estimation may not be equalto the frequency of measurement of the tool-to-tool disparity.

FIG. 22 shows an entire sequence for correcting a tool-to-tool disparityaccording to the sixth embodiment.

In this sequence, first, steps S41 to S50 shown in FIG. 4 and/or stepsS41 to S44, S51, S52, S48, S53, and S50 shown in FIG. 19 are performedat step S221 to estimate a factor(s) that has caused a tool-to-tooldisparity. Then, at step 222, the tool-to-tool matching controlapparatus 11 or 19 (or both) communicates with each microscope 10 or 18(or both) and thereby monitors the values of the image feature(s) and/ordevice state parameter(s) (exhibited by the microscope) that areaffected by the tool-to-tool disparity causing factor(s) estimated atstep 221. If the tool-to-tool disparity is found to be still larger thanthe maximum allowable value, the microscope control section 301 dcalibrates each microscope at step S224 by instructing the control unit215 of the microscope to adjust the device control parameters of themicroscope according to the monitoring results. More specifically, thetool-to-tool-disparity causing factor analyzing section 301 c or 301 edetermines the correlation between the dimension values (or CD values)measured by the microscopes and the values of each monitored imagefeature and/or device state parameter (exhibited by the microscopes) atstep S223. If their correlation is low, the (candidate) tool-to-tooldisparity causing factors must be newly analyzed to estimate a factorthat has caused the disparity again. In such a case, the process returnsto step S221 at which the tool-to-tool matching control apparatus 11 or19 (or both) estimates a factor(s) that has caused the tool-to-tooldisparity again. Then, at step 222, the tool-to-tool matching controlapparatus 11 or 19 (or both) monitors the values of the image feature(s)and/or device state parameter(s) (exhibited by the microscopes) that areaffected by the tool-to-tool disparity causing factor(s) determined atstep 221. If the tool-to-tool disparity is found to be still larger thanthe maximum allowable value (and the correlation calculated at step S223is high), the microscope control section 301 d calibrates eachmicroscope at step S224 by instructing the control unit 215 of themicroscope to adjust the device control parameters for the microscopeaccording to the monitoring results. These steps are repeated until theentire processing is complete.

As described above, the first to sixth embodiments of the presentinvention provide scanning electron microscopes, tool-to-tool matchingcontrol systems containing scanning electron microscopes, andtool-to-tool matching control apparatuses for scanning electronmicroscopes capable of quickly estimating a factor(s) that has caused atool-to-tool disparity between scanning electron microscopes andadjusting the microscopes based on the estimation results to reduce thedisparity. This leads to highly accurate control of the dimensions ofpatterns and hence to enhanced performance of the product.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all aspects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

The invention claimed is:
 1. A system for controlling a tool-to-tooldisparity between a plurality of scanning electron microscopes,comprising: a measuring unit for measuring a tool-to-tool disparitybetween plural scanning electron microscopes based on informationextracted from secondary electron images which are captured by imaging areference pattern, formed on a reference wafer on which a pyramidpattern, a repeated pattern, and a dense pattern having edges in allradial directions are formed as the reference pattern including line,hole, and dot patterns by using each of said plural scanning electronmicroscopes; a tool state monitoring unit for monitoring tool states ofeach of said plural scanning electron microscopes including at least oneof a deflector lens current and an objective lens current of each ofsaid plural scanning electron microscopes; and an output unit fordisplaying on a screen a relationship between said tool-to-tooldisparity between said plural scanning electron microscopes measured bysaid measuring unit and tool states of each of said plural scanningelectron microscopes monitored by said tool state monitoring unit;wherein said tool state monitoring unit monitors the tool states of eachof said plural scanning electron microscopes while imaging saidreference pattern by using each of said plural scanning electronmicroscopes.
 2. The system as claimed in claim 1, wherein each of saidplural scanning electron microscopes measures a dimension of a patternfrom a secondary election image of the pattern, and the reference waferused by the measuring unit is also used to detect a secondary electronimage for measuring a pattern dimension by each of said plural scanningelectron microscopes.
 3. The system as claimed in claim 1, wherein thereference wafer used by the measuring unit is the same with a referencewafer used for detecting a secondary electron image for measuring apattern dimension by each of said plural scanning electron microscopes.4. The system as claimed in claim 1, wherein said measuring unitmeasures said tool-to-tool disparity between said plural scanningelectron microscopes in accordance with image features extracted fromsaid secondary electron images.
 5. The system as claimed in claim 4,wherein said image features extracted from said secondary electronimages include at least one of an image brightness, an image contrast,an image noise, a pattern dimension, a pattern feature, a resolutionindicator value calculated from said image, and an axial alignmentindicator value calculated from said image.
 6. The system as claimed inclaim 1, wherein said tool states monitored by said tool statemonitoring unit further including device state parameters of componentsconstituting said scanning electron microscopes.
 7. The system asclaimed in claim 6, wherein said device state parameters include atleast one of an electron gun voltage and current or probe current, aretarding current and voltage, and a booster current and voltage.
 8. Thesystem as claimed in claim 1, further comprising: a microscope controlunit for selecting a device control parameter based on a tool-to-tooldisparity causing factor output from said output unit, and forautomatically adjusting said selected device control parameter for eachmicroscope by an amount sufficient to reduce said tool-to-tool disparitybetween said microscopes.
 9. The system as claimed in claim 1, furthercomprising: a database for storing a relationship between tool-to-tooldisparity causing factors and device control parameters for adjustingsaid tool-to-tool disparity causing factors; and a microscope controlunit for, based on said relationship stored in said database, selectinga device control parameter for said tool-to-tool disparity causingfactor output from said output unit, and for automatically adjustingsaid selected device control parameter for each microscope by an amountsufficient to reduce said tool-to-tool disparity between saidmicroscopes.